The present invention relates to a transparent optical device and its fabrication method. More specifically, the present invention relates to a transparent optical device having a porous layer on a surface to improve its optical characteristics.
Transparent optical devices, such as a photomask, an optical lens, an optical filter, an optical window, a prism, and a mirror, are, in many cases, fabricated by removing a damaged layer in the final processing step to obtain a mirror surface on a bulk substrate of the optical devices. This is particularly true for substrates having a high optical refractive index. The bare surface is usually coated by an antireflection layer. However, the conventional antireflection coating technique is not very useful for optical devices such as synthesized silica glass when this type of device is to be used with light in the deep ultraviolet wavelength region. The conventional coating is not very useful because synthesized silica glass has one of the lowest optical refractive indices in deep ultraviolet light among the widely used transparent optical materials, and a high energy photon of deep ultraviolet light often causes severe radiation damage to the antireflection layer which eventually decreases its optical transmissivity. This is known as a solarization effect.
In a phase shift photomask having a groove in the substrate surface in an area of either a main or a shifter pattern, which groove has been formed by dry etching in gaseous plasma of fluoro-hydrocarbon, it has been shown that a porous layer results in the groove and that the porous layer acts as an antireflection coating. However, the difference in transmissivity or reflectivity between areas which have been etched (main pattern) and those unetched (shifter pattern) by plasma gives rise to an undesirable optical interference at the boundary between the main and shifter patterns. Furthermore, higher transmissivity of the backside of the photomask reduces multi-reflection between the backside of the photomask and a highly reflective wafer surface, which can avoid an over-exposure of photoresist.
It is a primary object of the present invention to provide a transparent optical device having a porous layer on the surface of the substrate.
It is another object of the present invention to provide a method for fabricating a transparent optical device having a higher transmissivity in deep UV wavelength, without having to coat any foreign materials on the optical device.
It is still another object of the present invention to provide a transparent optical device in which an undesirable optical interference at a boundary between main and shifter patterns is minimized.
In experiments, an appreciable change was observed in optical transmissivity of deep ultraviolet light (UV) having a wavelength shorter than about 300 nm versus exposure time when a mirror-polished surface of a synthesized silica glass substrate was exposed to gaseous plasma of fluoro-hydrocarbon. In FIG. 1, three spectroscopic transmissivities are shown corresponding to three different exposure times: no exposure, 5 min. of exposure, and 10 min. of exposure, respectively. FIG. 1 indicates that the transmissivity increases for light below 300 nm in wavelength with increasing dry etching time, and that the transmissivity reaches a saturated value. This is due to the fact that a thin porous layer is formed on the bare surface of synthesized silica glass when the glass is exposed to gaseous plasma. This porous layer acts as an antireflection coating.
According to the present invention, there is provided a phase shift photomask having a synthesized silica substrate and a porous layer formed on both surface areas of the main and shifter pattern of the substrate by exposing these surface areas of the substrate to gaseous plasma of fluoro-hydrocarbon in a reactor chamber. By forming a porous layer on the both areas, the intensity of transmitted light is equalized at both sides between the main and shifter pattern, which produces a clear-cut pattern image for fine photolithography.